Issue 3205: opening bar check causes crash if \score contains the \midi block

Issue 3205: opening bar check causes crash if \score contains the \midi block

This derives Bar_check_iterator from Music_iterator rather than from
Simple_music_iterator.

In
\score {
  { | d }
  \midi { }
}

the bar check iterator is being called while only the \Global context
exists.  That causes Score_performer::one_time_step to be called
without getting Score_performer::prepare to be called previously,
probably because the Score context is created at the wrong time.  The
Score_performer is not prepared for this situation.

I have no idea how to fix Simple_music_iterator, why it exists in the
first place and is written like it is, and why this appears to work.